An ink jet recording apparatus has been known, which has a recording head for discharging ink as droplets onto a recording paper to print an image. The ink jet recording apparatus is provided with at least an ink container containing ink, to supply the ink from the ink container to the recording head. In an example, the recording head is provided with at least a nozzle and an oscillation plate driven by a piezoelectric element. Making use of pressure change in the nozzle, which is caused by oscillating the oscillation plate, the recording head sucks the ink from the ink container into the nozzle, and discharges the ink through an ink outlet of the nozzle.
Because the ink is a consumable material, the ink container is often formed as a cartridge that is removably attached to the ink jet recording apparatus, so the ink may be supplied conveniently. When the ink contained in the cartridge type ink container, hereinafter called the ink cartridge, is used up, the empty ink cartridge is replaced with another that is fully filled with the ink. In an ink cartridge loading section of the recording apparatus, an ink supply needle is disposed for supplying the ink from the ink cartridge to the recording head. The ink cartridge is connected to the nozzles of the recording head through an ink supply path, including the ink supply needle.
An ink jet recording apparatus disclosed in Japanese laid-open Patent Application No. 2003-300331 uses an ink cartridge that consists of a flexible ink bag and a case protecting the ink bag. If the ink is exposed to the air, the air will be solved in the ink, forming air bubbles in the ink, or some components of the ink react with oxygen, deteriorating the ink. To keep the air out of the ink, the ink cartridge uses the air-tight ink bag.
It is known in the art that the pressure inside the nozzle of the recording head, hereinafter called the nozzle internal pressure, is kept negative relative to the atmosphere, in order to prevent the ink leakage through the nozzle, which would otherwise be caused by the weight of the ink. Where the ink cartridge is placed above the recording head, the nozzle internal pressure is so raised by the weight of the ink contained in the ink cartridge, that it cannot keep the negative value relative the atmospheric pressure without any countermeasure. According to the above prior art, the air in a room between the ink bag and the case is sucked by a suction pump to reduce the pressure in the room, so that the nozzle internal pressure is kept negative relative to the atmospheric pressure.
As the ink in the ink bag is consumed, the pressure applied to the nozzle by the ink weight decreases, so the negative pressure in the nozzle would become too large if the case internal pressure is kept at the initial negative value. In that case, the ink discharged from the nozzle would be improperly reduced, lowering the print density improperly. To avoid this problem, the above-mentioned prior art suggests providing a pressure sensor for measuring the nozzle internal pressure, and controlling the amount of suction by the suction pump depending upon the measured nozzle internal pressure. Thereby, the nozzle internal pressure is kept in a proper range.
However, because the conventional method of controlling the nozzle internal pressure by controlling the pressure of the room between the ink bag and the case needs the suction pump, the apparatus for this method tends to have a complicated structure. So an alternative device that ensures stability of ink discharging property of the recording head without complicating the structure of the ink container has been desired.